Breathing is essential to human life, as well as voice and speech production. The diaphragm is the primary muscle of inspiration and its function is impaired in many diseases, such as amyotrophic lateral sclerosis, cervical spinal cord injury, and multiple sclerosis. Health care costs associated with the resultant blow to respiration and communication are staggering. As yet, no cortical therapeutic target has yet been identified to address diaphragm dysfunction. In fact, despite physiologic studies in cats and macroanatomical work in humans, the actual location of diaphragm motor cortex remains unknown. The ability to describe the cortical representation of the diaphragm is not only important for grasping volitional control of breathing, but also for understanding vocal control. Movements of the diaphragm are the first in a complex chain of events leading to vocalization and speech. These movements are in direct proportion to the amount of air needed to accomplish the upcoming communication task. Stated differently, the depth of an inspiration must come under exquisite preparatory control by the cerebral cortex. For animals capable of producing relatively sophisticated vocalizations, such as monkeys, it is possible that neural pathways directly from the brain to the motor neurons controlling the diaphragm exist. Monkeys are known to have these direct (cortico- motoneuronal) pathways from cortical motor areas in the hand region to the spinal cord, which bestow them with the ability to produce fractionated finger movements. Monkeys demonstrate voice and respiratory functions that are more closely analogous to that of humans than are other commonly studied animals. Thus, this project has the potential to (a) outline the precise location of the diaphragm motor cortex, (b) further reveal other premotor areas involved in diaphragm control, and (c) give insight regarding whether corticomotoneuronal projections exist for diaphragm control. Further, this project represents the first step toward clarifying whether two anatomically and functionally different parts of the diaphragm are each controlled uniquely by the cerebral cortex. The two parts-the costal and crural diaphragm-seem to play distinct roles not only in breathing, but also in gastroesophageal transport. This project will serve as a foundation for better understanding the unique neural pathways underlying the motor control of these two parts of the diaphragm. We propose to use rabies virus as a retrograde transneuronal tracer in order to characterize the cortical representation of the costal diaphragm in cynomolgous monkeys. The proposed investigations will fill a critical gap in our understanding of the neural substrates of diaphragm function, may serve as a springboard for identifying treatment options for patients with diaphragm dysfunction and failure, and will provide an invaluable fellowship training opportunity.